Epithelial sodium channels that reside at apical plasma membranes of cells of distal tubules of the kidney (and a wide variety of tight epithelia) are intimately involved in the control of whole body sodium balance by way of regulation of apical membrane permeability to sodium ions. This proposal focusses attention on the mechanisms involved in modulating the probability that a channel is open and the mechanisms that serve to control the density of channels through which sodium is absorbed into the cytoplasm of the cells. Whereas second messenger cAMP appears to be involved primarily in regulation of channel density, Ca++ appears to be involved in regulation of channel open probability. the hypothesis to be tested is that Ca++ acting by way of protein kinase C is the pathway mediating control of channel open probability. The combined power of methods of blocker-induced noise analysis and patch clamp analysis will be used to measure channel densities and open probabilities under conditions that increase/decrease cytosolic Ca++ activity (Ca++ transport at basolateral membranes and mobilization of intracellular Ca++) and that activate/inhibit protein kinase C activity directly. Cytosolic Ca++ activity will be measured by fluorescence imaging and the changes of Ca++ activity correlated in time with those of the channel densities and open probabilities.